oa South African Journal of Geology - The Lüderitz Alkaline Province, South West Africa II : metasomatism and assimilation in the contact aureole of the Granitberg Foyaite complex

Volume 78 Number 2
  • ISSN : 1012-0750
  • E-ISSN: 1996-8590



Reactions between the Outer Foyaite magma of the Granitberg Complex, and the sedimentary country rocks are striking. At sandstone contacts the magma has assimilated quartz-rich sedimentary material and a suite of pulaskites, peralkaline nordmarkites, and peralkaline granites have been generated by a complex process involving both assimilation and crystal fractionation. The sandstones have suffered Ca-Mg metasomatism in the contact zones and have been transformed into rocks containing perthite, diopside and sometimes quartz. At dolomite contacts, assimilation has resulted in the formation of a widely developed shonkinite with minor amounts of leucocratic, pegmatitic nepheline syenite and a suite of ijolite-melteigite rocks. Assimilation reactions have also generated a fenitizing fluid phase which is responsible for the observed fenitization in some of the contact rocks.

Metamorphic mineral assemblages in the calc-silicate rocks indicate that temperatures of 500 °C and greater prevailed in the contact zone, and that the fluid phase accompanying metamorphism had a low CO2 content.

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